In electrostatic printing and plotting systems, there is provided a print head which includes an array of electrodes which can be brought into close proximity with a writing surface and, when electrically energized, cause a charge pattern on the surface to which a toner can be applied to produce a visible image in the shape of the charge pattern. The print heads are often in the form of one or more rows of electrodes terminating in nibs or styli and retained in a support member, the styli being closely spaced to produce an effectively continuous print on an associated surface. Typically, 200 styli per inch are provided for graphic plotters.
Such print head have been fabricated by hand-wiring of an array of wires, the ends of which form the styli, as shown, for example, in U.S. Pat. No. 3,693,185. Another known technique of electrostatic print head fabrication is by printed circuit techniques such as shown in U.S. Pat. Nos. 3,267,485 and 3,718,936. Fabrication of a print head by hand-wiring is necessarily time-consuming, requires skilled labor, and is usually expensive and inefficient for quantity manufacture. Printed circuit fabrication techniques have not been wholly satisfactory in producing high performance print heads heretofore, as the production of high density styli without breaks or short circuits is extremely difficult using printed circuit materials and fabrication processes. Moreover, interconnection of electrode lines to the addressing network has been complicated and unreliable. In addition, the shape of the styli at the writing surface is preferably of square cross-section which is difficult to attain using plating or etching types of printed circuit processes.
In the fabrication of an electrostatic print head by printed circuit techniques, the achievable resolution is limited by the printed circuit board material, by reason of its relatively rough surface and the manner in which metal surfaces are formed on the printed circuit substrate board. Usually copper or other metal foil is adhesively secured to the printed circuit board substrate. Thus, a printed circuit board usually includes conductive foil adhered to the substrate surface by a layer of adhesive, and this structure cannot provide a very smooth or good surface. In addition, in printed circuit board technology, holes are not provided in the substrate until after lamination of the metal foil onto the substrate, since holes, if previously formed, would interfere with the lamination of the foil onto the substrate surface. Thus, printed circuit fabrication techniques require that conductive foil be provided on the substrate surfaces and holes thereafter formed through the substrate, with a conductive interconnection thereafter being provided in the holes to interconnect the top and bottom conductive surfaces. Such a conductive connection to the conductive board surfaces is subject to discontinuities and cannot provide a continuous conductive layer.
Another printed circuit fabrication technique is shown in U.S. Pat. No. 3,771,634, wherein a printed circuit board has holes drilled therethrough and then plated to form conductive interconnections between the upper and lower conductive surfaces. Alternative fabrication is suggested, employing a ceramic substrate, with the printed circuit techniques described.